Apparatus for temperature measurement

ABSTRACT

An apparatus for measuring the temperature of an infra-red source includes provision for modulating the output of an infrared sensitive device and for generating a signal in synchronism with peaks of the modulated output, said signal being used to control a switch so as to provide a d.c. signal porportional to the levels of said peaks. A circuit, which generates a function which is the inverse of the relationship between the source temperature and the d.c. signal, provides an output for the apparatus which has a linear relationship with the temperature.

United States Patent [1 1 Roberts 1 Oct. 23, 1973 [54] APPARATUS FORTEMPERATURE 3,441,846 4/1969 Petrohilos 73/362 AR X MEASUREMENT3,309,881 3/1967 Beerman 73/355 R X 7 3,161,771 12/1964 Engborg etal..... 73/355 R X [75] Inventor: Thomas Ernest Edwin Roberts, 3,350,56210/1967 Flint 73/355 R X 7 Birmingham, England Assignee: jtiseph ilucaslfndustries) Limited,

Birmingham, England amass CIRCUIT 3O Pay/5R sw/m/ Par/m- PrimaryExaminer-Richard C. Queisser Assistant Examiner-Frederick ShoonAttorney-Holman & Stern [57] ABSTRACT An apparatus for measuring thetemperature of an infra-red source includes provision for modulating theoutput of an infra-red sensitive device and for generating a signal insynchronism with peaks of the modulated output, said signal being usedto control a switch so as to provide a dc. signal porportional to thelevels of said peaks. A circuit, which generates a function which is theinverse of the relationship between the source temperature and the dcsignal, provides an output for the apparatus which has a linearrelationship with the temperature.

14 Claims, 3 Drawing Figures PAIENTEB um 2 3 191a SHEET 1 OF 3 EVEINVENTOR ATTORNEYS PAIENIEUucT 23 ma 3, 7 66. 781 sum 2 BF 3 INVENTOF?-Thomc.b Edwin @Obnh'th AT ORNEYS APPARATUS FOR TEMPERATURE MEASUREMENTThis invention relates to apparatus for temperature measurement, and hasas an object to provide such an apparatus in a convenient form.

According to the invention an apparatus for temperature measurementcomprises a means sensitive to infrared radiation and adapted to providea voltage signal whose magnitude is dependent on the level of the saidradiation, means for modulating the said voltage signal to provide analternating signal, a demodulator for the said alternating signal, thesaid demodulator comprising a means for generating a train of pulseswhich are substantially in synchronism with the peaks of the saidalternating signal and switching means for the alternating signaloperable by each of the said pulses, and linearising means operable bythe demodulated signal to provide an output which is linearly dependenton the temperature of a substance providing the infra-red radiation.

The invention also resides in the combination of an apparatus fortemperature measurement of the kind defined above with a temperaturemeasuring device having a high steady-state stability and means wherebythe output of the said device is modified by the said apparatus exceptin steady-state conditions.

An apparatus according to the invention will now be described by way ofexample and with reference to the accompanying drawings in which:

FIG. 1 shows such an apparatus diagrammatically,

FIG. 2 shows, somewhat diagrammatically, a section through part of theapparatus of FIG. 1, and

the block 11. The arrangement 30 includes a thermistor 31 secured to theblock 11 and responsive to the temperature thereof. Thermistor 31 formspart of a bridge circuit 32 whose output is supplied via a amplifier 33which includes a Schmitt trigger circuit to control a power switch 34.The arrangement 30 also includes a number of Peltier batteries 41 whichare supplied with power via the switch 34 and are secured in goodthermal contact with the block 11. The Peltier batteries 41 may bestacked so that the hot plate of one battery is in contact with the coldplate of an. adjacent battery. The block 11 may in this way bemaintained at a considerable temperature difference above or below FIG.3 shows an arrangement incorporating the appa I ratus of FIG. 1.

A photo-resistive device 10, sensitive to infra-red radiation has anexternal power source (not shown). Device 10 is mounted in an aluminiumblock 11 and has its sensitive area 10a directed towards a passage 12opening on to the outside of the block 11. Also mounted in the block 11is a gallium arsenide light emitting diode 14 and an adjacentphoto-resistive diode l directed towards the diode 14. A disc '16 isjournalled in the block 11 and coupled by a shaft 17 to a motor 18. Ablock 19 of insulating material is secured betweenthe block 11 and themotor 18. The disc 16 lies between the diodes l4, l5 and alsobetween-the device and the passage 12. Thejdisc 16 is formed with tenequi-spaced slots 16a having a mark-spaced ratio of one to one. Thedevice 10 and diodes 14, are positioned in the block 11 so that device10 and diode 15 are rendered, in use, conductive simultaneouslyQTheoutput of the device 10 is connected via a capacitor 20 and an amplifier21 to the primary of a transformer-22.

The secondary winding of the transformer 22 is connected to an input ofan electronic switching circuit 23. The output of the diode 15 isconnected via a Schmitt trigger circuit 24 and a monostablemultivibrator 25 to the switching circuit 23,.the output of themultivibrator providing the control signal for the circuit 23. Theoutput of the circuit 23 is connected via an amplifier 26 to alinearising circuit 27 whose function will later be described. Acapacitor 13 is connected between earth and the junction of circuit 23and amplifier 26. The output 28 of the circuit 27 provides the output ofthe apparatus.

The block 11 is formed with a recess 29 within which is mounted atemperature control arrangement 30 for the ambient temperature.

In use, the apparatus is mounted so that the passage 12 is directedtowards an object or substance whose temperature is to be measured.Rotation of the disc16 by the motor 18 causes infra red radiationfalling on the device 10 to be chopped. The output of the device 10 isthus a voltage waveform whose amplitude corresponds to the differencebetween maximum infra red input and no infra red input. Any inaccuracydue to a variable outputfrom the device 10 at zero infra-red input isthus eliminated. The DC component of the voltage waveform is eliminatedby thecapacitor- 20 to provide a signal which passes via the amplifier21 and transformer 22 to. the switch 23. The signal has at this stage awaveform shown at 22a.

Diode 15 is subjected to light from the diode-l4 and produces an outputof the form shown at 15a which is squared by the Schmitt trigger 24 tothe form shown at 24a.

Multivibrator 25 is controlled. by the waveform shown at 24a to producethe pulse train 25a, each pulse of which is in synchronism with thepeaks of the waveform 22a. The output, of the switching circuit 23 isthus a series of pulses whose levels correspond to the levels of thepeaks of the waveform 22a. The output pulses of the circuit 23 chargethe capacitor 13 toprovide the waveform shown at which is amplified inthe amplifier 26.The output V0 of the device 10, and hence the waveform13a has an exponential relationship to the temperature 'T of: the objector substance being measured, as shown by the curve 26a. Circuit 27effectively generates a function which is the inverse of the aforesaidexponential relationship, whereby the signal at the output 28 has alinear relationship, as shown at 27a with the temperature beingmeasured. For example, when vV0 K7, the inverse function is T ;Vo/K.

FIG. 3 shows an arrangement in which an apparatus 35, substantially aspreviously described, has its output 28 connected via a'resistor 36 anda capacitor 37 to an amplifier 38. A thermocouple 39 is connected via aresistor 40to the amplifier 38. In use the apparatus 35 and thermocouple39 are arranged to be subjected to the same temperature, whichtempieraturemayvary with time over a predetermined range, as shownat 41.The values of resistors 36, 40 are such that the output currents of theapparatus 35 and thermocouple 39 are substantially identical. The timeconstant of the thermocouple 39 for the aforesaid temperature range maybefldetermined. The value of the capacitor37 is such that the timeconstant of the resistor 36 and capacitor 37 is substantially equal tothe time constant of the thermocouple 39.

Radiation sensitive devices typically have short respouse-times, but lowsteady-state stability. This means that for a constant level ofradiation input, the output signals of these devices tend to vary. Thisdisadvantage offsets the advantageous short response-times of thesedevices. Conversely, thermocouple devices have good steady-statestability but disadvantageously long response-times.

The apparatus shown in FIG. 3 enables the short response-time of aradiation sensitive device to be combined with good steady-statestability of a thermocouple. Specifically, in the embodiment illustratedin FIG.

3, the thermocouple 39 which is responsive to the temperature of thesubstance giving out infrared radiation provides a first current signalthrough resistor 40. A second current signal initially substantiallyequal in magnitude to the first signal after a change of temperature ofsaid substance is provided through resistor 36. The second signal decaysto a zero value with a time constant substantially equal to that of thethermocouple. The first and second current signals are summed in a meansprovided therefor, and amplifier 38, for example, so as to provide areading which has the short response-time of the device 35 and the goodsteady-state stability feature of thermocouple 39.

The voltage waveform 28a which is produced at output 28 as a result ofthe temperature step shown at 41 produces, downstream of the capacitor37, the current waveform shown at 37a having a time constant t,. Thecurrent waveform from the thermocouple 39 resulting from the sametemperature step is shown at 40a. The magnitudes and time constants ofthe wave forms 37a, 40a are, as described, substantially identical andwhen summed in the amplifier 38 produce the waveform shown at 38a. Theoutput of the arrangement thus has a response time equal to that of theapparatus 35, and at the same time has a steady state stability equal tothat of the thermocouple 39.

Having thus described my invention what I claim as new and desire tosecure by Letters Patent is:

1. An apparatus for temperature measurement comprising a means sensitiveto infra-red radiation and adapted to provide a voltage signal whosemagnitude is dependent on the level of said radiation, an occultingshutter between said infra-red sensitive means and an 7 associatedsource of infra-red radiation, whereby said voltage signal comprises analternating signal, a demodulator for said alternating signal, saiddemodulator generating a train of pulses which are substantially insynchronism with the peaks of said alternating signal, said demodulatorincluding a pulse shaping circuit and a monostable multivibrator,switching means for the alternating signal operable by each of saidpulses, and linearising means operable by the demodulated signals toprovide an output which is linearly dependent on the temperature of asubstance providing the infra-red radiation.

y 2. An apparatus as claimed in claim 1 in which said means sensitive toinfra red radiation is a photoresistive device.

3. An apparatus as claimed in claim 1 in which said shutter comprises adisc formed with equi-spaced circumferential apertures, and whichincludes means for rotating said disc.

4. An apparatus as claimed in claim 3 in which said apertures are formedso as to provide a mark-space ratio of one to one for the shutter.

5. An apparatus as claimed in claim 1 in which said demodulator furtherincludes a light source, a photoresistive device and a shutter betweensaid source and said photo-resistive device.

6. An apparatus as claimed in claim 5 in which said shutter between saidsource and said photoresistive device is constituted by the shutterassociated with said infra-red sensitive means.

7. An apparatus as claimed in claim 5 in which the light source is agallium arsenide diode.

8. An apparatus as claimed in claim 1 in which said linearising meanscomprises a circuit for generating a function which is the inverse ofthe relationship between the temperature to be measured and the voltagesignal provided by said infrared sensitive means.

9. An apparatus as claimed in claim 1 which includes a metal block withwhich said infra-red sensitive means and said shutter are in thermalcontact.

10. An apparatus as claimed in claim 9 which includes temperaturecontrol arrangement for said block.

11. An apparatus as claimed in claim 10 in which said temperaturecontrol arrangement includes a Peltier effect device in thermal contactwith said block and switching means responsive to the temperature ofsaid block for regulation a current supply of said Peltier effectdevice.

12. An apparatus for temperature measurement comprising means sensitiveto infra-red radiation and adapted to provide a voltage signal whosemagnitude is dependent on the level of said radiation, means formodulating the impingement of said radiation on said radiation sensitivemeans, whereby said voltage signal is an alternating signal, ademodulator for said altemat ing signal, said demodulator comprisingmeans for generating a train of pulses which are substantially insynchronism with the peaks of said alternating signal, switching meansfor the alternating signal operable by each of said pulses, linearisingmeansoperable by the demodulated signal to provide an output which islinearly dependant on the temperature of a substance providing theinfra-red radiation, and a thermocouple disposed to sense thetemperature of said substance to providea first current signal whichchanges with'a known time constant for a given change in saidtemperature, the apparatus also including means responsive to the outputof said linearising means to provide, in response to said temperaturechange, a second current signal initially substantially equal inmagnitude to the magnitude of the first current signal after saidchange, said second current signal decaying to zero with a time constantsubstantially equal to that of said thermocouple, and means for summingthe first and second current signals. 1

13. An apparatus as claimed in claim 12 in which said means responsiveto the output of the linearising means comprises a resistance andcapacitance arrangement having a time constant equal to that of saidthermocouple.

14. An apparatus as claimed in claim 12 in which said summing means isan amplifier.

t III

1. An apparatus for temperature measurement comprising a means sensitiveto infra-red radiation and adapted to provide a voltage signal whosemagnitude is dependent on the level of said radiation, an occultingshutter between said infra-red sensitive means and an associated sourceof infra-red radiation, whereby said voltage signal comprises analternating signal, a demodulator for said alternating signal, saiddemodulator generating a train of pulses which are substantially insynchronism with the peaks of said alternating signal, said demodulatorincluding a pulse shaping circuit and a monostable multivibrator,switching means for the alternating signal operable by each of saidpulses, and linearising means operable by the demodulated signals toprovide an output which is linearly dependent on the temperature of asubstance providing the infrared radiation.
 2. An apparatus as claimedin claim 1 in which said means sensitive to infra red radiation is aphoto-resistive device.
 3. An apparatus as claimed in claim 1 in whichsaid shutter comprises a disc formed with equi-spaced circumferentialapertures, and which includes means for rotating said disc.
 4. Anapparatus as claimed in claim 3 in which said apertures are formed so asto provide a mark-space ratio of one to one for the shutter.
 5. Anapparatus as claimed in claim 1 in which said demodulator furtherincludes a light source, a photo-resistive device and a shutter betweensaid source and said photo-resistive device.
 6. An apparatus as claimedin claim 5 in which said shutter between said source and saidphotoresistive device is constituted by the shutter associated with saidinfra-red sensitive means.
 7. An apparatus as claimed in claim 5 inwhich the light source is a gallium arsenide diode.
 8. An apparatus asclAimed in claim 1 in which said linearising means comprises a circuitfor generating a function which is the inverse of the relationshipbetween the temperature to be measured and the voltage signal providedby said infrared sensitive means.
 9. An apparatus as claimed in claim 1which includes a metal block with which said infra-red sensitive meansand said shutter are in thermal contact.
 10. An apparatus as claimed inclaim 9 which includes temperature control arrangement for said block.11. An apparatus as claimed in claim 10 in which said temperaturecontrol arrangement includes a Peltier effect device in thermal contactwith said block and switching means responsive to the temperature ofsaid block for regulation a current supply of said Peltier effectdevice.
 12. An apparatus for temperature measurement comprising meanssensitive to infra-red radiation and adapted to provide a voltage signalwhose magnitude is dependent on the level of said radiation, means formodulating the impingement of said radiation on said radiation sensitivemeans, whereby said voltage signal is an alternating signal, ademodulator for said alternating signal, said demodulator comprisingmeans for generating a train of pulses which are substantially insynchronism with the peaks of said alternating signal, switching meansfor the alternating signal operable by each of said pulses, linearisingmeans operable by the demodulated signal to provide an output which islinearly dependant on the temperature of a substance providing theinfra-red radiation, and a thermocouple disposed to sense thetemperature of said substance to provide a first current signal whichchanges with a known time constant for a given change in saidtemperature, the apparatus also including means responsive to the outputof said linearising means to provide, in response to said temperaturechange, a second current signal initially substantially equal inmagnitude to the magnitude of the first current signal after saidchange, said second current signal decaying to zero with a time constantsubstantially equal to that of said thermocouple, and means for summingthe first and second current signals.
 13. An apparatus as claimed inclaim 12 in which said means responsive to the output of the linearisingmeans comprises a resistance and capacitance arrangement having a timeconstant equal to that of said thermocouple.
 14. An apparatus as claimedin claim 12 in which said summing means is an amplifier.